Fuel Consumption
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Thread: Fuel Consumption

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    Default Fuel Consumption

    Having finally found a reasonable quality set of performance data for the DW10AETD engine I spend a little time to build a model of the 406 HDi sedan that estimates fuel consumption under different conditions. The simple example is the car running on a flat road, which matches well to measured data.

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    Fuel Consumption-steady.png
    And a larger plot of the fuel consumption across different gradients.
    Fuel Consumption-consumption.jpg
    The uncomplicated result is that keeping the rev as low as possible will save the most fuel, interestingly the steady state data would suggest even keeping under 1500rpm and the turbo boost. The fun side of the data is that going up hill its most efficient to stay up at the speed limit as the engine efficiency improves so much with the added load that fuel economy stays pace with or beats the increased aerodynamic drag.

    The next step will be to look at pulse and glide, but there isn't much literature available on the dynamic characteristics of turbochargers. My best guess at the moment is that when lifting off the throttle very little if any of the energy in the turbine is able to be recovered, anyone have some ideas?

    Also I have a good academic engineering document for the DW10AETD engine if anyone wants a copy of it (6MB pdf).
    Last edited by hypermiler; 8th November 2014 at 04:55 PM.
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    Keeping the revs as low as possible makes sense. The 'slower' the engine is going, the less fuel is being used. Your finding that trying to stay at the speed limit going up hill is interesting.
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    Quote Originally Posted by 2353 View Post
    Keeping the revs as low as possible makes sense. The 'slower' the engine is going, the less fuel is being used. Your finding that trying to stay at the speed limit going up hill is interesting.
    Lots of gears is how it's done. Also turbo charging and/or supercharging to keep the low revs volumetric efficiency optimum. At least in petrol engines.

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    Very interesting charts. Do they take aerodynamic drag into account? If so, I would have thought that the steady-state fuel consumption curve would be steeper with increasing speed than it is. The figure of about 4 l/100km at 120 km/h seems a bit unlikely in the real world (at least according to the trip computer on my 406 HDI).
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    Keeping the engine in its optimum operating range with least throttle percentage will return you best economy.

    Install a vacuum gauge and drive on that, aiming for the highest reading.
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    Watching a vacuum gauge....Has to be as bad as texting

    4l/100km....not enough fun

    10l/100km.....starting to warm up

    30l/100km....now we are stoking
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    Quote Originally Posted by sfrawley View Post
    Very interesting charts. Do they take aerodynamic drag into account? If so, I would have thought that the steady-state fuel consumption curve would be steeper with increasing speed than it is. The figure of about 4 l/100km at 120 km/h seems a bit unlikely in the real world (at least according to the trip computer on my 406 HDI).
    All losses are modelled, it was surprising how flat it was. Up at 120km/h the average would be above 4.5l/100km in steady driving on a flat road with no braking.

    I've measured several points on the curve to confirm the data and it's surprisingly accurate, even predicting the specified 0-100km/h time of 12 seconds. If you're not getting low 4's on the highway have you still got the aerodynamic package (plastic tray) under the front half of the car?
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    Quote Originally Posted by COL View Post
    Keeping the engine in its optimum operating range with least throttle percentage will return you best economy.
    but that's not actually true, pulse and glide pushes the engine efficiency up 50% or more. Had the computer die and lost some of the modelling so I will have to get back to it in a week or two when I have the time, but the short result is you can save 20-30% with pulse and glide.
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    Quote Originally Posted by sfrawley View Post
    Very interesting charts. Do they take aerodynamic drag into account? If so, I would have thought that the steady-state fuel consumption curve would be steeper with increasing speed than it is. The figure of about 4 l/100km at 120 km/h seems a bit unlikely in the real world (at least according to the trip computer on my 406 HDI).
    Yeah, it seems a bit odd as the aerodynamic drag curve starts to sweep upwards dramatically above 80km/h.

    Obviously, the less aerodynamic the car, the faster the curve sweeps up.

    The torque peak of that motor plateaus between about 1700 and 2100rpm which may explain the efficiency up to 110 but I would've thought the tail end of the blue line would start to curve upwards by 120kmh as it drops of it's torque peak, but the graph seems to indicate the opposite.

    I'd imagine that keeping the revs at 1700 would be a good sweet spot for that motor based on the power curve, which seems to translate into the real world.


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    The torque is limited by the ecu map to 250nm from 1400rpm upward with a tolerance band of about +/- 10nm when built, so it doesn't have a peak as such.

    There aren't specific efficient engine speeds or accelerator demands (what some might call throttle in the old language) to target, the efficiency is a vehicle specific map from available gears and the power demand as shown in the complicated plot of the first post.

    Taking just one aspect of the car such as the drag and making incorrect decisions/assumptions from that is exactly what this work has been about avoiding and disproving.

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    Maybe I'm being pedantic, but drag is not an aspect, it is a force that rises exponentially with speed.

    To overcome drag requires power. The more drag, the more power is required to maintain a speed.

    I'm not trying to disprove anything as physics dictate that energy useage must increase proportionally to drag - (unless the engine is not running efficiently at the required speed). The slight flattening of your graph would indicate either an error in the data - possibly a tail wind that was not accounted for or similar.

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    Quote Originally Posted by mistareno View Post
    Maybe I'm being pedantic, but drag is not an aspect, it is a force that rises exponentially with speed.
    How about google dictionary for some pedantry:
    as·pect/ˈaspekt/


    noun

    1. a particular part or feature of something.

    Drag is one aspect of the vehicle dynamics, an aspect included in the modelling work presented here.
    Quote Originally Posted by mistareno View Post
    To overcome drag requires power. The more drag, the more power is required to maintain a speed.

    I'm not trying to disprove anything as physics dictate that energy useage must increase proportionally to drag - (unless the engine is not running efficiently at the required speed). The slight flattening of your graph would indicate either an error in the data - possibly a tail wind that was not accounted for or similar.
    The DW10AETD engine has some rather unusual brake specific fuel consumption characteristics which end up with the interesting results above, you can get additional power in certain circumstances for very little additional fuel. Its not a simple matter of power bring proportional to fuel, most obviously exhibited in the very low speeds where the engine is not optimised for. You can see the very detailed efforts the engineers put into developing this engine and car when its matched up against realistic use scenarios.
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    Quote Originally Posted by driven View Post
    Watching a vacuum gauge....Has to be as bad as texting
    No different to keeping an eye on the other gauges
    Regards Col

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    Quote Originally Posted by COL View Post
    Quote Originally Posted by driven View Post
    Watching a vacuum gauge....Has to be as bad as texting
    No different to keeping an eye on the other gauges
    505 Econoscope to the rescue!
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    Quote Originally Posted by COL View Post
    No different to keeping an eye on the other gauges
    But a bit pointless on a diesel as it would always read zero.
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    Quote Originally Posted by sfrawley View Post
    But a bit pointless on a diesel as it would always read zero.
    That would be correct. But they work well on a petrol engine, and can also tell you the state of tune of the engine.
    Regards Col

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    Quote Originally Posted by hypermiler View Post
    All losses are modelled, it was surprising how flat it was. Up at 120km/h the average would be above 4.5l/100km in steady driving on a flat road with no braking.

    I've measured several points on the curve to confirm the data and it's surprisingly accurate, even predicting the specified 0-100km/h time of 12 seconds. If you're not getting low 4's on the highway have you still got the aerodynamic package (plastic tray) under the front half of the car?
    The underbody tray is there but I'm a bit doubtful about the accuracy of the trip computer. There is no easy way to check the instantaneous fuel consumption but the average over a tankful is always 0.5 to 1.0 l/100km high (when compared to the manual calculation). If the displayed instantaneous fuel consumption figure has a similar error then your graphs look more realistic.

    Btw, thanks for the reports - a lot of heavy reading there!
    Stephen
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    Quote Originally Posted by driven View Post
    Watching a vacuum gauge....Has to be as bad as texting

    4l/100km....not enough fun

    10l/100km.....starting to warm up

    30l/100km....now we are stoking
    Maximum Fun; 100l/100km via an LPG powered V8 VK Rally Commodore, 325 kW at the wheels!
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    Got back to this with some spare time having learnt a lot more about turbo dynamics, the huge amount of energy stored in the spinning turbine is quite disturbing. Presented is the relative fuel use compared to steady driving all on a flat road at the same average speed.
    Fuel Consumption-pulseimprove.png
    Interestingly the amount of speed change in each pulse didn't really affect the results until you reach large changes like accelerating up 50km/h before starting the glide again. Friction losses at the higher rpms make the gliding in neutral a better choice for highway speeds. These lower fuel consumptions disappear once the engine loads up from driving up an incline, with the savings dropping to 0 at only a 4% grade.

    Despite the sensible gains from this sort of driving style its completely inappropriate for a turbo diesel as the constant changes in speed of the turbine will fatigue the rotor, failure of that part is catastrophic.

    So the ideal strategy seems to be not being afraid of the throttle except when the extra speed will result in braking.
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    Quote Originally Posted by bluey504 View Post
    Maximum Fun; 100l/100km via an LPG powered V8 VK Rally Commodore, 325 kW at the wheels!
    Brendan.
    Somewhat missing the fun factor but I can beat that. Falcon wagon towing a trailer with about 3 tonne of rowing boats on it up the old freeway out of Adelaide.

    70 Litres of fuel for 35km = 200L/100km!
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    You have left one graph out. The graph of falling petrol prices. That dictates that science can be thrown out of the window for the time being and replaced with sheer hedonism for the summer. Given that the faster you go, the cooler the airstream is, you save on aircon usage and its detrimental effect on fuel consumption.

    Science is for when the the fuel price rises and you have to think your way out of poverty.

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    Talking about gliding (neutral) or angel gear as we used to call it reminds me of the Rover 90 which had a free wheel arrangement but only for use on the open road as of course there was no engine braking. One simply pressed the accelerator until the motor caught up with the road speed;excellent idea.
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    I remember the Rover Free-Wheel knob, also another fuel saving device, the Laycock de Normanville electric overdrive. I had a brother-in-law with a 1951 bullet nosed Studebaker that had three speeds with a factory overdrive too, but I'm not sure how that worked.....yes I do! A knob on the left of the dash in this pic is marked OD. I wonder what would happen if I pulled it.....?
    Attached Thumbnails Attached Thumbnails Fuel Consumption-studebaker-dashbord.jpg  
    Last edited by Kim Luck; 30th November 2014 at 01:37 AM.
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    Memories,memories,Kim;also had a Graham (1939 model)deemed to be the quickest 39 American car of the time;had supercharger and a large knob which actuated the overdrive but you had to virtually stop to get out of o'drive or suffer the sparks and smoke,Andy.

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    Yep,my Landlord has a pair of '40 Supercharged Grahams,one Green & one Blue,He's hand them longer than I've been alive...and I'm 50. Like Alhantos,said,the O.D.could ONLY be activated @ very low speeds,the Supercharger,with it's Ball Bearing construction,was another high-maintaince item. Many American Cars of this era had O.D. as 'an option.

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